The present invention relates to a technique including an image correcting device configured to create an output image through geometrical correction of an input image.
In some application fields including a monitoring camera and a video door phone, there has been a demand in recent years for obtaining images of a wide visual field. As a solution to meet the demand, a video camera, with a fisheye lens, or the like is commercially available which captures an image in a range with a right/left angle of view of approximately 180 degrees. However, in general, in an image captured by such a device, a straight line in a subject space (for example, a straight line in the real world) is greatly distorted. Thus, there is a demand of performing geometrical correction to a captured image which allows a degree of distortion to be reduced.
There are various fisheye lenses according to different design concepts, including various applicable projection methods such as equidistance projection, equisolid angle projection, and orthogonal projection. In any of these projections, distortion of a line in an image captured with a fisheye lens (hereinafter, referred to as “a fisheye image”) is completely correctable by applying perspective transformation based on a projection model and a design value/values and/or a given parameter/parameters estimated by a conventional technology, as long as the angle of view to be represented by an image subjected to the correction, that is, an image after correction, is smaller than 180 degrees.
FIG. 9A illustrates an example of a fisheye image. FIG. 9B illustrates an example where the fisheye image is corrected by applying perspective transformation such that the angle of view in a horizontal direction is ensured to be wide. As shown in FIG. 9B, as a result of the correction through perspective transformation, although a portion to be represented as a line is properly corrected to be linear, a marginal portion of the image is extremely expanded with an emphasis on perspective, causing a feeling of strangeness of the image. This is a problem caused by a significant difference between the angle of view represented by the image as a result of the correction and the angle of view when seeing that image.
JP 2010-067172A discloses a technique developed to solve the problem. In the technique disclosed by JP 2010-067172A, transformation expressions are designed such that characteristics of the transformation varies from those of a transformation similar to a perspective projection (perspective transformation), which is applied for the central portion of an image, to those of a transformation similar to an orthogonal projection, which is applied for the marginal portion of the image. Thus, a fisheye image inputted is corrected such that a line in a subject space appears linear in a vicinity of the center of the image, while an angle of view of 180 degrees is ensured in both the right/left direction and the top/bottom direction.